The enzyme, which exists in all vertebrates, decarboxylates ethylmalonyl-CoA, a potentially toxic compound that is formed in low amounts by the activity of EC 6.4.1.2 (acetyl-CoA carboxylase) and EC 6.4.1.3 (propanoyl-CoA carboxylase). It prefers the S isomer, and can decarboxylate (R)-methylmalonyl-CoA with lower efficiency. cf. EC 4.1.1.41 (methylmalonyl-CoA decarboxylase).